In a hard disk drive (HDD) including a magnetic disk, a magnetic head records or reads data on or from the magnetic disk with a gap between the magnetic head and the magnetic disk, or while floating over the magnetic disk. However, touchdown is likely to occur in which the magnetic head cannot be floated smoothly due to the temperature, moisture or altitude of use environment of the HDD, a portion projected from the magnetic disk or the like to bring the magnetic head into contact with the magnetic disk.
When touchdown occurs, the magnetic head floats high, its positioning decreases in accuracy or the like, thereby causing a recording error and a reading error. It is thus required to control floating the magnetic head over the magnetic disk with high accuracy to prevent touchdown from occurring under use environment of the HDD.
Conventionally, there is a touchdown detection method using an AC output of a head-disk interface (HDI) sensor. The HDI sensor is provided in a magnetic head, includes a resistive element and is able to detect touchdown by variations in resistance value of the HDI sensor. There is another touchdown detection method using servo position error information in place of the HDI sensor. In both methods, touchdown should be detected with high accuracy.